Plants are known (EP-A-0,795,616) which are intended for the continuous heat treatment of metal strip, in which the strip to be treated travels through an oven consisting of a plurality of heat-treatment sections, especially heating sections, temperature-hold sections, annealing sections, cooling sections, etc. In this publication, the present proprietor has recommended the use of heating and/or cooling enclosures or chambers in which there is an atmosphere consisting of a hydrogen-based gas mixture, the hydrogen content of which is greater than the values usually permitted so as to increase the cooling rate. Thus, according to this prior technique, a gas mixture, especially a nitrogen/hydrogen gas mixture, having a hydrogen content which is greater than 15% and which may reach 50%, i.e. greater than the explosibility limit value for this gas, may be used as the atmosphere in the cooling enclosure.
In the cooling enclosure of the abovementioned plants, the recycled atmosphere gas is blown onto the continuously moving metal strip, after it has been cooled especially with the aid of gas/water heat exchangers, the composition of the atmosphere gas, and more particularly its high hydrogen content, making it possible to obtain very rapid cooling rates of the order of 100.degree. C. per second. The plants therefore comprise a circuit of ducts for recirculating the N.sub.2, H.sub.2 atmosphere gas, one or more fans for continuously circulating this cooling gas in the recirculation ducts, and gas/water exchangers.
Given the very high hydrogen content of the atmosphere gas used during the rapid cooling, it is necessary to take great precautions to ensure that the plant is safe; indeed, a leak, however tiny, of the atmosphere gas into the environment cannot be tolerated, nor can ingress of air into the recirculation ducts.
The present invention aims to provide a solution to this safety problem, more particularly in the case in which all the cooling-gas circulation ducts have compensators which allow the ducts to expand. In the event of a break in a corrugation of a compensator, or of a leak in the latter, it is absolutely necessary to prevent the ingress of air, i.e., of oxygen, into the cooling-gas recirculation circuit or any leakage of this cooling gas into the environment.
The present invention solves the above mentioned problem by placing a flexible membrane impermeable to the cooling gas around each compensator of the recirculation duct, a means of detecting a break in a corrugation of a compensator being furthermore provided, this means immediately interrupting the flow of the cooling gas in the circuit for recirculating the latter.
According to one embodiment of the present invention, the flexible membrane impermeable to the cooling gas is made in the form of a textile membrane, for example a membrane made of glass-fiber-reinforced silicone.
According to the invention, the system for detecting a break in a corrugation of the compensator, or a leak in the latter, is achieved by means of a flow of a pressurized inert gas, preferably nitrogen, in the space lying between the compensator and the impermeable flexible membrane which surrounds it, means being furthermore provided for detecting any variation in the pressure of this inert gas as a result of the cooling gas leaking out through the compensator, so as to stop the flow of said gas in the recirculation ducts.
According to one embodiment of the invention, each compensator is provided with a metal casing, in the form of a circular ring surrounding the corrugations of the compensator, this casing being provided with a circular opening closed off by the flexible and impermeable membrane, and it has an inlet and an outlet for the inert gas which is introduced under pressure between the walls of the casing and the compensator, the means for detecting a pressure variation in the inert gas being placed on the casing.
According to the present invention, the means for immediately stopping the flow of the cooling gas in the recirculation ducts for this gas may be of the type described in the patent application filed on the same day as the present application by the same proprietor and entitled: "Improvements made to rapid-cooling ovens for metal strip", i.e. a register system, placed upstream of the exchangers, in the flow area for the rapid-cooling gas, and a system of actuators with a very short response time, i.e. of less than 5 seconds, which control the said register system so as to close the flow area under the control of the detection means.
Other features and advantages of the present invention will emerge from the description given below with reference to the appended drawings which illustrate an example of an embodiment thereof, this example being devoid of any limiting character.